Redox-activated acrylic structural adhesives are well known articles of commerce which are used commercially for bonding metal to metal, and for cross bonding two dissimilar substrates materials. Acrylic structural adhesives have found growing use in the automotive industry where the adhesive bonding of metal parts is replacing welding and mechanical fastening techniques. However, these applications give rise to unique requirements not easily met by previously available adhesives. These requirements include high bond strength and improved failure mode.
Redox cured acrylic adhesives typically comprise a mixture of one or more olefinic reactive monomers such as methyl methacrylate or methacrylic acid and curing agents, with cure or polymerization being effected through an ambient temperature redox initiation mechanism employing an oxidizing agent and reducing agent. Typical reducing agents are tertiary amines. Di-substituted derivatives of aniline are disclosed in U.S. Pat. No. 4,421,879 (3,-4-disubstituted aniline). U.S. Pat. No. 5,932,638 discloses p-halogen with 3,4-disubstitution, and is shown to overcome the incidence of air inhibition.
U.S. Pat. No. 5,641,834 and U.S. Pat. No. 5,710,235, both incorporated herein by reference, disclose combinations of isocyanate capped olefinic-terminated polyalkadiene. The composition also includes a free radical-polymerizable monomer such as an olefinic monomer and, optionally, a second polymeric material. In a preferred embodiment the composition is an adhesive that also includes a phosphorus-containing compound and an ambient temperature-active redox catalyst.
Additional important features of acrylic adhesives are surface tack and open time. As used herein, “surface tack” means the amount of adhesive on an exposed surface of the applied adhesive that does not undergo curing. Such uncured adhesive can be transferred to other parts of the assembly or to the application equipment resulting in increasing clean-up costs. A common cause of surface tack is referred to as “air inhibition” since atmospheric oxygen is a powerful inhibitor of free radical reactions. Accordingly, the amount of surface tack can be measured by determining the thickness of any uncured adhesive on the surface.
In typical applications of two-part reactive adhesive systems, the two parts are mixed together, the mixed material is applied to a first substrate for bonding then a second substrate is contacted to the adhesive-applied first substrate. The time required for such mixing, applying and contacting is referred to herein as “open time”.
A variety of approaches have been used in the art to overcome residual tackiness in the adhesive which has been subject to air inhibition. U.S. Pat. No. 6,294,249 discloses a pre-adhesive which is combined with a packaging material and further polymerized to form a packaged hot melt adhesive composition which is subsequently used in bonding. The adhesive comprises a) 50 to 100 parts by weight of a polymerizable component comprising at least one acrylic or methacrylic ester of a non-tertiary alkyl alcohol in which the alkyl group contains 1 to 20 (e.g., 3 to 18) carbon atoms; (b) 0 to 50 parts by weight of a polymerizable component comprising at least one modifying monomer, other than said acrylic or methacrylic ester, copolymerizable with component (a), the sum of (a) and (b) amounting to 100 parts by weight; (c) an effective amount of a polymerization initiator; and (d) an effective amount of a chain transfer agent. The polymerization initiator is preferably a photoinitiator or a thermal initiator.
U.S. Pat. No. 5,997,682 relates to a method for fabricating a structural assembly comprising the steps of providing at least two articles, each article having a surface; applying a dual-cure adhesive mixture to at least a portion of at least one of the surfaces; joining the at least two articles such that the adhesive mixture is positioned between the at least two articles; at least partially curing a first portion of the adhesive mixture by irradiating at least one of the articles with electron-beam energy such that a structural assembly is formed wherein the at least two articles are adhesively bonded together by the at least partially cured first portion of the adhesive mixture and a remaining portion remains uncured by the electron-beam energy while the first portion is curing; and allowing the remaining portion of the adhesive mixture to become cured thermally.
It would be advantageous to provide an adhesive which does not require any additional heat to initiate the cure, especially where the assembled structure is made from materials that distort under application of heat, such as most rigid thermoplastics. It would also be advantageous to avoid the use of special reducing agents to overcome problems of air inhibition, while providing a rapid elimination of surface tackiness in the adhesive which is exposed to air.